1. Field of the Invention
The present invention relates to a proton conductive solid electrolyte membrane for solid polymer fuel cells. To be more concrete, the present invention relates to a solid electrolyte membrane for methanol fuel cells (or direct methanol fuel cells that will be also called DMFCs) or hydrogen fuel cells.
2. Description of the Related Art
Solid electrolyte membranes are indispensable materials for electrochemical elements such as solid polymer fuel cells, temperature sensors, gas sensors, electrochromic devices, etc. Among these uses, solid polymer fuel cells are expected to play a major role in future new energy-technologies. When a solid electrolyte membrane is used for a fuel cell, it is often called a proton conductive membrane, since it plays a role of conducting protons.
Among solid polymer fuel cells, methanol fuel cells are expected to be promising as a power source for electric automobiles, since methanol can be supplied as a liquid fuel, similar to gasoline. Also, due to its ease of handling, methanol fuel cells are expected to be promising as an energy source for electric/electronic portable devices.
Methanol fuel cells are classified into two types: reforming-type cells in which methanol is converted into a gas mixture mainly composed of hydrogen, using a reformer; and DMFC's that directly use methanol without using a reformer. Among them, practical applications of DMFCs in portable electric/electronic devices are highly expected, because compact and light-weight devices can be realized since no reformer is necessary in these cell systems.
Organic polymer materials having sulfonic acid groups, carboxylic groups, phosphoric groups and the like are used as solid electrolyte membranes of fuel cells. Conventionally, perfluorosulfonic acid polymers such as Nafion® membrane of Du Pont and Dow membrane of Dow Chemical have been widely used as such organic polymer materials.
However, the problem with these membranes is that although they have excellent proton conductivity, there is a strong tendency for the methanol, which has a high affinity for water, to permeate from the anode side to the cathode side (methanol crossover), when they are used as solid electrolyte membranes of DMFCs. When methanol crossover occurs, the supplied fuel (methanol) directly reacts with oxygen at the cathode, thus making it unable to output energy as electricity.
Proton conductive membranes that are not affected by the water content in them are considered to have low methanol crossover. However, for example, in the case of polybenzimidazoles (PBIs) doped with strong acids such as phosphoric acid, although the methanol crossover is low, there is a problem that inorganic dopants leach out from the membrane into water/methanol solutions.
Attention has been focused on sulfonated polyphenylene ethers, polyether ketones, polyimides, polybenzoxazoles, polybenzothiazoles and the likes as materials capable of controlling methanol crossover (see, for example, Japanese Patent Application Laid-open No. 2002-201269 (Claims, and paragraph No. 2-4)). The major problem of these polymers is that sufficient proton conductivity could not be obtained, since they do not form appropriate ion channel structures. Also, they undergo chemical degradation in strong acidic and oxidative conditions, through the CH bonds of these materials.
At present, perfluorinated resins such as Nafion are indispensable materials as binders of electrode layers of fuel cells. Accordingly, it is essential to develop an electrolyte membrane with some elastic properties in order to improve the compatibility with the aforementioned perfluorinated binders, thereby to realize good adhesion between the membrane and the electrode layers. Furthermore, polymer structures with least number of CH, CH2, CH3, and CO2R groups are necessary to improve the oxidative stability of the membrane, or in other words, it is essential to develop materials with molecular structures having perfluoroalkylenes, fluorinated phenyl groups, sulfones, carbonyl groups, or the like. Moreover, in order to control methanol crossover, introduction of a cross-linked structure is indispensable.
Regarding fluorinated proton conductive materials other than Nafion, sulfonated perfluorocyclobutane-containing polymers having proton conductivities superior than that of Nafion, and their fuel cell properties (PEM) have been disclosed (see U.S. Pat. No. 6,559,237 (claims). Furthermore, use of porous perfluorocyclobutylene polymers to support the electro-conductive particles of the electrode binders and to facilitate gas permeation in electrodes of fuel cells have been disclosed in U.S. Pat. No. 5,620,807 (claims). However, they have not disclosed the use of perfluorocyclobutylene polymers as proton conductive membranes.
In addition, according to the reported method for synthesizing perfluorocyclobutane-containing sulfonated polymers, first perfluorocyclobutane-containing polymers have been synthesized from non-sulfonated monomers and then they have been converted to sulfonated perfluorocyclobutane-containing polymers by direct sulfonation using chlorosulfonic acid. It should be mentioned that in this method of sulfonation, it is difficult to control the degree of sulfonation depending on the molecular structure, and sometimes the sulfonation reaction does not proceed at all.